Low oxygen environments are a hallmark of solid tumors and the transcription of many hypoxia-responsive genes needed for survival under these conditions is regulated by the heterodimeric transcription factor HIF-1 (hypoxia-inducible factor 1). The transcription factor HIF-1 is critical for initiating adaptive responses to low oxygen environments and maintaining cellular homeostasis. HIF-1 regulates the transcription of numerous hypoxia-responsive genes, including ones that modulate glycolysis and glucose flux, and those associated with vasodilation and angiogenesis such as vascular endothelial growth factor. Overexpression or dysregulation of HIF-1 function has been implicated in tumor progression, metastasis, resistance to chemotherapies, and poor clinical outcomes for a variety of tumor types.
Inhibition of HIF-1 activity has potential therapeutic applications for a variety of tumor types. HIF-1 is a heterodimer of α- and β-subunits, and the transcriptional activity of this complex is regulated by either the accumulation or turnover of the HIF-1α monomer. Under normoxic conditions HIF-1α is rapidly recycled in a process that involves hydroxylation of specific proline residues, followed by ubiquitination and proteasomal degradation of the monomer. When oxygen tension is low, HIF-1α accumulates in the nucleus where it dimerizes with the constitutively present HIF-1β subunit. This allows recruitment and binding of the transcriptional coactivator p300, a multidomain protein that not only plays a crucial role in HIF-1 activation but also has intrinsic histone acetyl transferase and polyubiquitin ligase activities.
Activation of HIF-1 requires binding of its α-subunit (HIF-1α) to the transcriptional co-activator protein p300. Inhibition of the p300/HIF-1α interaction is an attractive approach to suppress HIF-1 activity. The essential binding interaction between p300 and HIF-1 that facilitates hypoxia-induced transcription involves the cysteine histidine-rich domain 1 (CH1) of p300 and the C-terminal transactivation domain (C-TAD) of HIF-1α.
Small molecule inhibitors of specific protein-protein interactions are quite rare, due in part to the large surface contact areas that are involved. Many of the small molecules that can inhibit protein-protein interactions have poor specificity or low potencies, with inhibitory concentrations often observed in the millimolar range. Block et al. designed and synthesized a series of dimeric epidithiodiketopiperazines, and the lead compound was shown to selectively inhibit the p300/HIF-1α interaction (J. Am. Chem. Soc. 2009, 131:18078-18088). In cellular assays it downregulated the expression of hypoxia-responsive genes, and in a murine model it significantly reduced in vivo tumor growth (Dubey et al., J. Am. Chem. Soc. 2013, 135:4537-4549). This study served as a proof of principle that validated small molecule targeting of the p300/HIF-1 interaction to inhibit tumors. The actinobacterial metabolite novobiocin, an aminocoumarin glycoside, was shown to disrupt the p300/HIF-1α interaction by directly binding to the HIF-1α C-TAD domain (Wu et al., PLoS One 2013, 8:362014).
HIF-1 also plays a role in other conditions besides tumors, such as malaria and inflammatory conditions. Inflamed and injured tissue may exhibit hypoxia in addition to other biomarkers of inflammation. Inflammatory mediators, such as proinflammatory cytokines, as well as certain bacterial and viral compounds, can activate HIF-1 to produce an inflammatory response.
Accordingly, HIF-1 inhibition is believed to inhibit tumor progression, metastasis, and resistance to chemotherapies, inhibit malarial infection, and/or ameliorate conditions including a hypoxia-mediated inflammatory response.